1. Field of the Invention
The invention relates to a device for forming X-ray images by means of an X-ray image converter which includes a photoconductor which at least partly absorbs the X-rays and is provided on a substrate which acts as an electrode, which device includes means for charging the photoconductor with a given polarity so that an electric field having a defined direction is generated in the photoconductor. The invention also relates to an X-ray imaging apparatus including such a device.
2. Description of the Related Art
An ideal photoconductor is an insulator when it is not exposed. It is merely in the case of an exposure or an irradiation by X-rays that it becomes conductive, i.e. the more so as the radiation intensity is higher. Thus, at the irradiated locations the charge density produced by a previous charging operation is reduced in conformity with the dose incident at those locations. The two-dimensional charge pattern thus produced on the surface of the photoconductor, essentially corresponding to the spatial distribution of the X-ray dose (latent image or charge image) is converted into electric signals by a read unit, which signals can be amplified, filtered, digitized and stored. The signals are thus rendered suitable for digital image processing.
From EP-A 0342760 which corresponds to U.S. Pat. No. 4,939,759, it is known that the so-called memory effect can occur due to defect spots in the photoconductor. Due to these defect spots a given conductivity remains in their vicinity after irradiation; consequently, structures of the preceding X-ray image appear as artefacts when the next X-ray image is formed. The memory effect is more pronounced as the dose of the preceding exposure was higher. Therefore, it becomes manifest only in the X-ray images; it does not have a disturbing effect in the case of X-ray fluoroscopy where a substantially lower X-ray dose is generated per individual image. In the known device the artefacts caused by the memory effect are eliminated or reduced by way of a correction based on software.